"It can't and won't be the case that you go for an extension without the agreed reforms," Finance Minister Wolfgang Schäuble's spokesman told reporters.
"Both are inextricably linked."
Greek public television has reported that Athens would send a letter on Wednesday to Jeroen Dijsselbloem, the head of the Eurogroup, to request a six-month extension.
However, Greece has signalled it wants to extend the loan agreement without the loathed commitments agreed to by the former centre-right government in return for its €240-billion bailout, which expires on February 28.
Schäuble had appeared unmoved on Tuesday night by the news.
Interviewed by ZDF television on Tuesday night, Schäuble said “there is no loan agreement. This is an aid programme for Greece.
“Greece wants more credit, but there have to be conditions... it's about the principle of helping them help themselves.”
Schäuble argued that Greece had been on the path back to financial stability, but “now the people are being instigated with false promises... hopes are being raised and they're being told that others are to blame for their problems.
“Of course I'm annoyed that people in Greece are having a hard time, and they've been living for years with the consequences of their former governments”.
Nevertheless, he said there was no quick way out of the crisis for Greece, “but a long, tough path of recovery and improvement".
Eurozone finance ministers set five conditions at their talks Monday for continuing their financial support to Athens, including a pledge not to reverse previously accepted reforms.
Other conditions include that Athens should not undertake new reforms that would burden Greece's public finances, and a commitment by Athens to reimburse its creditors.
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An extension request had not yet been officially presented by midday Wednesday, the finance ministry spokesman Martin Jäger told reporters.
Negotiations are running up against a deadline, as any plan to extend the bailout will need to be approved by several national parliaments - including Germany - before February 28th.